This invention, which is used in an apparatus such as an electrophotographic copying machine, relates to a device for giving a tension to a beltlike photoreceptor, to a belt device running in an image transfer portion with carrying a paper or to a belt device as an intermediate transfer member for transferring a toner image formed on a photoreceptor to a paper. Especially, the invention relates to a device capable of absorbing vibration and uneven driving speed of the belt in accordance with rotation of a driving roll and a tension roll.
As the electrophotographic copying machine, in an image forming apparatus which forms copied papers with the electrophotographic method, a photoreceptor dram is used as an image carrier. The image forming apparatus has a mechanism which transfers the toner image formed on the image carrier onto a paper and fuses the image on the paper through a fusing device, to output the copied paper. On the other hand, the image forming apparatus sometimes employs another mechanism in which a beltlike photoreceptor is used as the image carrier, the elements constituting the electrophotographic system are provided with respect to the beltlike photoreceptor, and then the toner image formed on the photoreceptor is transferred to the paper to obtain the copied paper.
As described above, for example, an electrophotographic copying machine having a beltlike photoreceptor has been known as disclosed in Japanese Patent Unexamined Publication No. Hei. 1-312577. A color electronic copying machine has been described in Japanese Patent Unexamined Publication No. Sho. 59-168467, which discloses that the paper is transported in accordance with a plurality of image transfer portions under a condition in which the belt member supports the paper. The color electronic copying machine, described in the prior art, employs a plurality of image forming units for forming a color image, which are arranged along the belt member carrying the paper, and the color toner images are transferred from the image forming units to the paper superimposingly during the movement of the paper.
As described above, in the image forming apparatus with the belt member, the belt member is put across a driving roll and a roll for giving the tension, and the belt member is driven at constant speed. However, even if length and width of the belt of the apparatus are constituted strictly, the belt member can not move at constant speed under conditions in which both an axis of a roll member for driving the belt and a circumference portion contacting with the belt are not formed with the same diameter circle exactly in a direction of the belt width.
The driving roll and the tension roll are produced by machine processing, molding processing and grinding the roll member, however, an axis center of the circumference portion contacting with the belt does not always agree with that of an axis member. In addition, in case of driving the roll and the belt member, it can not be avoided that a vibration is occurred on the belt member in accordance with an eccentricity quantity of the roll. In the belt apparatus, the driving roll is held to be arranged at a fixed position to a supporting frame, and the tension roll is supported though a tension giving member, such as a spring, giving a constant tension to the belt.
Now, presuming that the tension roll is arranged immovably and is held by the frame with giving constant tension. The vibration generates and has an influence to the belt member, and the belt expands and contracts in accordance with the eccentricity of the tension roll. The influence of expansion and contraction of the belt depends on materials and size of the belt, position of the belt, the eccentricity quantity of the roll member, and twining angle of the belt to the roll. In addition, in an apparatus such as a general belt driving apparatus in which the belt member is put across the driving roll and the tension roll so that the belt is moved by driving the driving roll, the tension roll is constituted to be movable in a tension giving direction by employing a spring or the like, so that the influence according to the eccentricity of the roll member is reduced. However, as described above, even if the tension giving device is provided movably, a variation of speed at a transporting surface or an action surface of the belt affects the image forming operation because of transport load of the tension giving device and a spring constant of a spring member for giving tension.
On the other hand, for example, as disclosed in Japanese Patent Unexamined Publication No. Sho. 59-168467, the color electronic copying machine employs a method in which color toner images formed by a plurality of image forming units are transferred sequentially and superimposingly to the paper. In the apparatus described on the prior art, two rolls provided at both sides of the belt member are used as driving rolls and both rolls are driven synchronously. However, as described above, even if a roll having the tension giving device and driving roll are constituted to have the same diameter and driven synchronously, the driving radiuses to the belt are different at each position of the rolls, and then expansion and contraction of the belt appears in different manners. Accordingly, when the color images are transferred to the paper superimposingly under a condition of holding the paper on the belt member, the color toner images slip out of place to deteriorate the image quality of the color copy.